Defects in integrated circuit chips may arise during a patterned wafer fabrication process. Defect information may be obtained from wafer inspection tools to improve production yield. Two types of defect detection tools are commonly used: (1) optical inspection tools, which illuminate a wafer using optical light, and (2) scanning electron beam inspection tools, which illuminate a wafer using electrons.
Optical inspection systems provide throughput on the order of a few hours for a 300 mm wafer inspection task, utilizing parallel data acquisition. However, the resolution of optical inspection tools is restricted by the diffraction limit imposed by the (relative to electrons) large wavelength of photons. Currently, optical inspection systems are not able to identify all of the defects at fourteen nanometer and smaller design scales.
The diffraction limit imposed by scanning electron beam inspection tools is significantly lower than that of optical tools because the wavelength of electrons is on the order of angstroms. Scanning electron beam inspection tools provide resolution on the order of a few nanometers, which is enough resolution to detect defects at fourteen nanometer and lower patterned wafer design scales. Unfortunately, the throughput of scanning electron beam inspection tools is low because imaging data is obtained in a serial manner during the scanning process. Depending on the signal-to-noise ratio required for the inspection process, the data acquisition rate of scanning electron beam inspection tools is commonly from 100 mega pixels to about one giga pixel per second. Currently, it takes more than a week to inspect a 300 mm wafer with a ten nanometer pixel size using a conventional scanning electron beam inspection system.
A multiple electron beam inspection system utilizing parallel data acquisition may increase the throughput and shorten the time to inspect a wafer. One parallel approach utilizes a common electric current coil to generate magnetic excitations at an array of holes on a magnetic pole piece, creating an array of magnetic electron focusing lenses. The current coil has only one axis of symmetry, which means that only one lens in the array can be excited symmetrically. The other lenses are excited asymmetrically because the lens optical axis does not overlap with the geometric axis of the excitation coil. This symmetry mismatch causes magnetic field distortion along the optical axis, degrades resolution, and introduces non-uniform performance among different lenses. Another approach creates a small current-driven electron magnetic focusing lens unit, and assembles an array of such lens units together. The lens excitation is limited by the current density in a limited space; resolution of the lens is limited as a result. Yet another approach utilizes non-shielded permanent magnets and shared magnetic conductors to create a focusing lens array. All of these prior approaches either have two focusing lens fields, complicating deflection and detection system design, or they exhibit a strong leakage field along the optical axis, which degrades resolution and field of view.